1. Field of the Invention
This invention relates to a method for grinding with controlled grinding force and a grinding machine which is suitable to use the method, and more particularly to a grinding method and a grinding machine therefor which monitors the sharpness of a grinding wheel in operations, changes command value for the grinding force corresponding to the detected sharpness of the grinding wheel, and effectively stabilizes the cycle time of the machine.
2. Discussion of the Background
In the prior art grinding systems with controlled grinding force, the grinding force normal to a workpiece is maintained constant during grinding operations irrespective of the changes in sharpness of the grinding wheel. FIG. 7 shows a control system of the prior art grinding method. In finished grinding, a grinding force S is set at a predetermined value higher than a threshold force (marginal grinding force) irrespective of the changes in the grinding wheel diameter, the actual grinding force is detected by a grinding force detector 3, a control device 4 is used to control a feed motor 5 in a manner to make the detected grinding force agree with said value S, and a feed slide of the wheel or a workpiece at a grinding device 6 is moved.
In the aforementioned prior art grinding method, the grinding force is set constantly at a value higher than the threshold force during finish grinding in order to prevent the surface of the workpiece from being tapered, and hence grinding is conducted with the set force constantly. However, the threshold force changes depending on the diameter of a grinding wheel to affect the sharpness thereof, which in turn greatly changes the grinding cycle time to thereby pose a critical problem in stabilization of productivity. More specifically, the threshold force generally becomes larger when the wheel diameter increases while it becomes smaller when the wheel diameter decreases. FIG. 2 shows the relation between the grinding force F and the grinding velocity V wherein reference letters and numerals F.sub.01 and F.sub.02 denote respectively the threshold force with a large wheel diameter and the threshold force with a small wheel diameter. The wheel sharpness coefficient K is obtained by multiplication with a constant of the gradient of the linear line in the graph. The sharpness of a grinding wheel is good when the threshold force F.sub.0 is smaller and the coefficient K is greater. For example, when the finish grinding force is set at the value F.sub.f in the graph, the grinding velocity becomes V.sub.1 when the grinding wheel diameter is large, and V.sub.2 (V.sub.2 &gt;V.sub.1) when the grinding wheel diameter is small. Assuming that the set value for grinding force F.sub.f is optimal at the start to the quality of the workpiece with the grinding velocity of V.sub.1 with a larger diameter wheel, the diameter of the wheel decreases as the grinding operation proceeds. Then, the grinding velocity V.sub.2 becomes so high that grinding of the workpiece to the right roundness at the finish becomes inferior, and wear and tear of the wheel is accelerated to thereby increase errors in the axial forms. Conversely, if the finish grinding force is set at the level F.sub.f' so as to obtain the optimal velocity V.sub.2' or V.sub.1 with a smaller wheel diameter, F.sub.f ' becomes smaller than the threshold force F.sub.01 for the larger wheel diameter, and therefore grinding becomes impossible. There has been proposed a method to prevent fluctuation in cycle time by changing the grinding force corresponding to the grinding wheel diameter, but the sharpness of the wheel changes due to the changes in the condition of a truer, and in such a case, prediction is impossible and the change of grinding force in advance is also impossible.
There is also known a grinding machine which measures the dullness of the grinding wheel and controls the truing of the grinding wheel based on the measured result of the wheel dullness in order to conduct grinding operations at the maximum efficiency without causing thermal damages on the workpiece (Ref. U.S. Pat. No. 3,745,710). According to said patent, it is proposed that the dullness of a grinding wheel is measured by continuously monitoring either the relation between the grinding force and the feed rate or the relation between the grinding force normal to the wheel surface and the grinding force tangential thereto. There is also known a grinding method which measures the normal resistance vectors between the grinding wheel and the workpiece and changes the feed rate relative between the grinding wheel and the workpiece depending on the intensity of the normal resistance vectors.